This process of manufacture results in an attrition resistant sorbent for use in gas desulfurization processes. In the process of desulfurization of coal gases, mixed metal oxides have been used to remove hydrogen sulfide (H2S) in the temperature range of 370 to 650xc2x0 C. (700 to 1200xc2x0 F.). Although the reactivities of zinc titanates and other zinc based sorbents are acceptable in this temperature range, the physical and mechanical integrity of the sorbents needs to be improved so that the sorbents can be used over repetitive cycles of sulfur absorption and sorbent regeneration. For moving-bed reactor configurations, the mixed metal sorbents are prepared from powders which are extruded into pellets and calcined to achieve a balance point of desired reactivity and pellet strength as measured by attrition and crush strength tests. Unfortunately, this balance between reactivity and pellet strength is difficult to achieve and many highly reactive pellets become mechanically weak and fall apart after several cycles due to repetitive phase transformations that occur during repeated cycles of absorption and regeneration. During sulfur removal, zinc-based oxides become zinc sulfide as shown in Equation 1.
Zn2TiO4+2H2Sxe2x86x922ZnS+TiO2+2H2Oxe2x80x83xe2x80x83(Equation 1)
During regeneration, the sulfide form is oxidized under diluted air back to the zinc oxide or zinc titanate structure as shown in Equation 2.
2ZnS+TiO2+3O2xe2x86x92Zn2TiO4+2SO2xe2x80x83xe2x80x83(Equation 2)
However, a certain level of zinc sulfate may be formed in the regeneration process. Zinc sulfate is detrimental because it reduces the sulfur absorption capacity of the sorbent, does not release the sulfur as a desired sulfur dioxide (SO2) product, and is often re-released to the clean gas in the subsequent absorption step. Additionally, the appearance and build-up of zinc sulfate results in the structural weakening of the pellets and leads to the attrition of pellet materials.
To prevent pellet weakening and eventual degradation, additives can be included in the zinc titanate formulations to stabilize phases, allow plasticity, and reduce mechanical stresses during phase transformations between the zinc sulfide layer and regenerated (i.e., oxidized) states of the sorbent pellet.
U.S. Pat. No. 5,494,880 (xe2x80x9c""880xe2x80x9d) issued to Siriwardane claims sorbent pellets containing zinc oxide, titanium oxide, silica gel, Bentonite and calcium sulfate for removing H2S from a gas stream. U.S. Pat. No. 5,86,503 (xe2x80x9c""503xe2x80x9d) issued to Siriwardane includes in its claims a mixed metal oxide with an inert material selected for particle size with the purpose of creating void space between the pellet components. The void space increases the porosity of the sorbent and is said to provide additional durability. The current application differs from ""880 in that the sorbent resulting from the current process involves zinc titanate compounds to which metal oxides have been added resulting in a chemical substitution of zinc and/or titanate ions within the zinc titanate compound for the metal of the oxide that was added.
U.S. Pat. No. 5,188,811 (xe2x80x9c""811xe2x80x9d) issued to inventor Ayala described the usefulness of mixed metal oxides, including zinc titanate compounds, in sulfur removal and simultaneous ammonia decomposition catalysts when combined with molybdenum. The inventor""s current process adds molybdenum oxide only in combination with magnesium oxide and this combination is only one of several potential metal oxide additives described here.
Gupta, et al. was granted a series of patents including, U.S. Pat. No. 5,254,516 (xe2x80x9c""516xe2x80x9d); U.S. Pat. No. 5,714,431 (xe2x80x9c""431xe2x80x9d); U.S. Pat. No. and 5,97,835 (xe2x80x9c""835xe2x80x9d), that include zinc titanate materials in sorbents used in fluidized bed reactors for removal of sulfur species. Gupta""s ""431 patent is for desulfurization applications where zinc titanate sorbents are of a spherical shape of uniform size and high reactivity composition adjusted to an average particle size of less than 300 microns . The ""835 patent claims a process for making attrition resistant spray dried fluidizable particles of between 100 and 400 microns. The ""516 patent starts with zinc titanate to which is added small amounts of CoO and MoO3. Claimed in the patent are additions to zinc titanate including: Groups VIB, VIIB and VIII metal compounds and Groups IA and IIA alkali and alkaline earth metal compounds. In addition to these groups of compounds. the ""516 patent also requires the addition of an inorganic and organic binder. The current process focuses on sorbents for use in moving-bed reactors while Gupta""s sorbents are for fluidized beds. Gupta also requires the addition of both an organic and inorganic binder whereas the current process has no such requirements.
The key to preventing pellet degradation due to phase transformation is the ability to modify the chemical composition and pelletization of the sorbent so that it can withstand phase transformations. The present sorbent is attrition resistant due to the chemical substitution of another metal (including magnesium, calcium, yttrium, zirconium, hafnium, copper, and molybdenum plus magnesium) for the zinc or titanium atoms within a zinc titanate compound. The method described here results in an attrition resistant sorbent when actual chemical substitution of the metals occurs and in the case of the metal (added as a metal oxide) being incorporated into the structure of the sorbent as a grain growth inhibitor.
The primary object of this invention is to provide a process that results in a modified zinc titanate sorbent for use in gas desulfurization that has increased attrition resistance, increased mechanical strength and where the sorbent""s reactivity for sulfur removal has not been diminished. The modified sorbent has zinc and/or titanate ions within the original zinc titanate compound substituted with the ions from a metal oxide additive.
The resulting sorbent has had 2% of the original zinc ions substituted by magnesium ions, or a magnesium ion substituted for about 11 mol % as magnesium oxide. Or the resulting sorbent has had about 2% of the original titanium ions substituted for zirconium ions.
An alternate object of the invention is to incorporate a grain growth inhibitor into the crystalline structure of the zinc titanate sorbent by the process described here. The source of the grain growth inhibitor is a metal oxide.
This process results in an improved sorbent for use in desulfurization of coal gas streams by increasing attrition resistance and mechanical strength while maintaining the sorbent""s reactivity for sulfur removal. The process starts with a zinc titanate compound to which is added, by milling in an aqueous medium, a metal oxide and then about 3 weight percent silica. The mixture is calcined, crushed, sieved and formed into pellets for use in a moving-bed reactor.
Metal oxides suitable for use as an additive in this process include: magnesium oxide, magnesium oxide plus molybdenum oxide, calcium oxide, yttrium oxide, hafnium oxide, zirconium oxide, and cupric oxide. The resulting sorbent has a percentage of the original zinc or titanium ions substituted for the oxide metal of the chosen additive. The resulting sorbent may in addition to, or instead of, the ionic substitution have the metal oxide added incorporated into the sorbent""s crystalline structure as a grain growth inhibitor.